1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus capable of obtaining information on a pressure gradient in a blood vessel or elasticity of the blood vessel based on reception signals of ultrasonic echoes reflected within an object to be inspected. The present invention further relates to a method of measuring a pressure gradient and a method of measuring blood vessel elasticity, to be used in the ultrasonic diagnostic apparatus.
2. Description of a Related Art
As indexes of blood vessel elasticity, a pulse wave velocity, a stiffness parameter β, and so on are known. In equipment in practical use, ABI (Ankle Brachial Pressure Index) measurement of measuring pulse wave propagation velocities in an arm and a foot, CAVI (Cardio Ankle Vascular Index) measurement of estimating a stiffness parameter β by using the propagation velocities, and so on are used.
However, these values are indexes weighted by fixed factors, but are not estimation of an absolute value of blood vessel elasticity. Further, for measurement of the pulse wave propagation velocities, measurement between sufficiently spaced two points is necessary and the arm and the food are selected therefor, and thus, the values do not express the elasticity of a local area of the blood vessel but only express average values in parts over significantly wide ranges. In addition, the pulse wave measurement is blood pressure measurement using a cuff (a band wrapped around an arm in a manometer), and there is a problem about stability of the measurement.
On the other hand, there has been an attempt to evaluate properties of a blood vessel by measuring IMT (Intima Media Thickness) of an artery such as a carotid artery by using ultrasonic waves. This method is simple but there is a discussion that a change of IMT appears as a result of arteriosclerosis and do not necessarily have a correlation with blood vessel elasticity.
Further, in recent years, there has been an attempt to measure blood vessel properties by FMD (Flow Mediated Dilatation) examination of measuring changes of an inside radius of an artery or IMT before and after interruption of blood flow. However, the measurement is far from simple measurement because measurement environments are strict such that it is necessary to keep a patient at rest, and blood pressure measurement must be performed at the same time with ultrasonic measurement, and so on. Although it is possible to estimate a blood vessel elastic modulus according to its definition by measuring the change of a blood vessel wall thickness such as IMT in a systolic phase and a diastolic phase, it is necessary for the purpose to know the magnitude of an applied force to change the blood vessel wall thickness. What dilates a blood vessel is its inner pressure, and therefore, it is appropriate to use the change in blood pressure as the change in force. However, blood pressure measurement must be performed at the same time with ultrasonic measurement, and an assumption is necessary that the blood pressure is internal pressure at ultrasonic measurement. As described above, the FMD examination has a problem of unreliability of measurement and a problem of complication of measurement.
As a related technology, Japanese Patent Application Publication JP-P2004-41382A discloses an ultrasonic diagnostic apparatus for more correctly measuring local blood pressure by using ultrasonic waves. The ultrasonic diagnostic apparatus includes transmitting and receiving means for transmitting and receiving ultrasonic waves to and from an object to be inspected, blood vessel size waveform information computing means for obtaining local size waveform information with respect to a specific local part in a blood vessel within the object, which information represents temporal changes of a size thereof, based on reception signals obtained by transmission and reception of ultrasonic waves, a manometer to be mounted on a body surface of the object, for measuring a systolic blood pressure and a diastolic blood pressure, and estimating means for estimating local blood pressure waveform information with respect to the specific local part, which information represents temporal changes of a local blood pressure thereof, by converting the local size waveform information based on the systolic blood pressure and the diastolic blood pressure measured by the manometer. The ultrasonic diagnostic apparatus is characterized in that the estimating means executes a computation of obtaining the local blood pressure waveform information by providing the systolic blood pressure and the diastolic blood pressure to a nonlinear function for estimating the local blood pressure waveform information from the local size waveform information.
According to JP-P2004-41382A, although the local blood pressure waveform information representing the temporal changes of the local blood pressure can be obtained based on the local size waveform information obtained with respect to the specific local part in the blood vessel within the object, it is necessary to measure the systolic blood pressure and the diastolic blood pressure by using the manometer, and the measurement becomes complicated.
Further, JP-P2002-209857A discloses a method of measuring a blood vessel elastic modulus by which a blood vessel elastic modulus is easy to be actually measured by using a manometer and an ultrasonic diagnostic apparatus and more credible blood vessel elastic modulus can be measured than a conventional one. The method of measuring a blood vessel elastic modulus includes the steps of measuring first blood pressure pi1, a blood vessel inside radius a1, and a blood vessel outside radius b1, measuring second blood pressure pi2, a blood vessel inside radius a2, and a blood vessel outside radius b2, and obtaining a blood vessel elastic modulus E according to the following expression.E=[pi2{a22/(b22−a22)}−pi1{a12/(b12−a12)}]/{(b2−b1)/(b2+b1)}
According to JP-P2002-209857A, although the blood vessel elastic modulus can be obtained based on the two types of blood pressure and the corresponding blood vessel inside radii and blood vessel outside radii, it is necessary to measure the two types of blood pressure by using the manometer, and the measurement becomes complicated.